Originally posted by Randybaker99
1) Cheap option:
Ebay - search for "600 rear derailleur" or something like that. OK, I just tried it and there were 7 hits.
To see how much you will end up paying click the "Completed Listings" checkbox on the left hand side of the screen. Prices range from $5 - $25.
2) Less cheap option:
replace the rear wheel with one that has a 8 or 9 speed cassette, then add a modern derailleur.
3) Expensive option:
New bike
My vote is for option number 3!